ricevuto il 31 Agosto 2012
Summary. — We present new results on charmonium and charmonium-like states from the BaBar experiment located at the PEP-II asymmetric energy e+e−collider at the SLAC National Accelerator Laboratory.
PACS 13.66.Bc – Hadron production in e−e+ interactions. PACS 14.40.Lb – Charmed mesons.
PACS 14.40.Pq – Heavy quarkonia.
PACS 13.25.Gv – Decays of J/ψ, Υ, and other quarkonia.
1. – Charmonium spectroscopy
The charmonium spectrum consists of eight narrow states below the open charm threshold (3.73 GeV) and several tens of states above the threshold. Below the threshold almost all states are well established. On the other hand, very little is known above the threshold, there are several new “Charmonium-like” states that are very difficult to accommodate in the charmonium spectrum.
The B-factories are an ideal place to study charmonium since charmonium states are produced in four different processes:
– B decays, charmonium states of any quantum numbers can be produced
– Two photon production: in this process two virtual photons are emitted by the colliding e+e− pair (e+e− → e+e−γ∗γ∗ → e+e−(c¯c)), charmonium states with
JP C= 0±+, 2±+, 4±+, . . . , 3++, 5++. . . can be produced.
– Initial State Radiation (ISR): where a photon is emitted by the incoming electron or positron (e+e−→ γc¯c), only states with JP C= 1−− are formed
– Double charmonium production: in this process a J/ψ or a ψ(2S) is produced together with another charmonium state.
c
2. – Study of the process γγ→ J/ψω
The charmonium-like state Y(3940) was first seen by Belle [1] and then confirmed by BaBar [2] in the same B meson decay mode, but with lower mass and smaller width compared to the Belle results.
In a re-analysis [3] of the BaBar data which used the complete Υ(4S) data sample, the precision of the Y(3940) measurements was improved and evidence for the decay X(3872)→ J/ψω was reported. This confirmed an earlier unpublished Belle claim for the existence of this decay mode [4]. The latter was based on the behaviour of the invariant
π+π−π0 mass distribution near the X(3872), whereas the BaBar result is obtained directly from a fit the the J/ψω mass distribution.
A subsequent paper from Belle [5] reports the observation in γγ→ J/ψω of a state, the X(3915), with mass and width values similar to those obtained for the Y(3940) in the BaBar analysis [2].
The BaBar analysis of the process γγ→ J/ψω has been performed in order to search for the X(3915) and the X(3872) resonances via the decay to J/ψω, using a data sample of 519 fb−1. Figure 1 presents the reconstructed J/ψω invariant-mass distribution after all the selection criteria have been applied. We perform an extended maximum-likelihood fit to the efficiency-corrected spectrum. A large peak at near 3915 MeV/c2 is observed
with a significance of 7.6 σ. The measured resonance parameters are m[X(3915)] = (3919.4± 2.2 ± 1.6) MeV/c2, Γ[X(3915)] = (13± 6 ± 3) MeV. The measured value of the
two-photon width times the branching fraction, Γγγ[X(3915)]× B(X(3915) → J/ψω) is (52± 10 ± 3) eV and (10.5 ± 1.9 ± 0.6) eV for the spin hypotheses J = 0 and J = 2, respectively, where the first error is statistical and the second is systematic. In addition, a Bayesian upper limit (UL) at 90% confidence level (CL) is obtained for the X(3872), Γγγ[X(3872)]× B(X(3872) → J/ψω) < 1.7 eV, assuming J = 2.
3. – Study of the process γγ→ ηcπ+π−
This analysis has been studied for the first time and is performed to search for reso-nances decaying into ηcπ+π−, using a data sample of 474 fb−1. The ηc was reconstructed via its decay to K0
SK+π−, with KS0 → π+π−. The signal yield for each X resonance is extracted from a two-dimensional fit to m(K0
SK+π−) and m(KS0K+π−π+π−). Figure 2 presents the two-dimensional fits around each of the resonances. No significant signal is
Fig. 2. – Distributions of (a,c,e) m(KS0K+π−) and (b,d,f) m(KS0K+π−π+π−) with the fit func-tion overlaid for the fit regions of the (a,b) χc2(1P ), (c,d) ηc(2S), and (e,f) X(3872), X(3915) and χc2(2P ). The vertical dashed lines in (f) indicate the peak mass positions of the X(3872), X(3915) and χc2(2P ).
TableI. – Results of the γγ → ηcπ+π− fits. For each resonance X, we show the peak mass
and width used in the fit; the product of the two-photon partial width Γγγ and the X → ηcππ
branching fraction, and the 90% CL upper limits on this product.
Resonances MX (MeV/c2) ΓX (MeV) ΓγγB (eV) UL
Central Value χc2(1P ) 3556.20± 0.09 1.97± 0.11 7.2+5.5−4.4± 2.9 15.7 ηc(2S) 3638.5± 1.7 13.4± 5.6 65+47−44± 18 133 X(3872) 3871.57± 0.25 3.0± 2.1 −4.5+7.7−6.7± 2.9 11.1 X(3915) 3915.0± 3.6 17.0± 10.4 −13+12−12± 8 16 χc2(2P ) 3927.2± 2.6 24± 6 −16+15−14± 6 19
observed in any of the fits. Table I summarizes these results. ULs are obtained on the branching fractionsB(ηc(2S)→ ηcπ+π−) < 7.4% andB(χc2(1P )→ ηcπ+π−) < 2.2% at 90% CL.
4. – Search for the Z1(4050)+ and Z2(4250)+
Belle reported the observation of two resonance-like structures, Z1(4050)+ and
Z2(4250)+ in the study of ¯B0→ χc1K−π+, both decaying to χc1π+ [6].
BaBarstudied the same final states [7] to search for the Z1(4050)+ and Z2(4250)+ decay into χc1π+ in ¯B0 → χc1K−π+ and B+ → KS0χc1π+ (this mode is analyzed for the first time) where χc1 → J/ψγ, using a data sample of 429 fb−1. The χc1π+ mass distribution, background-subtracted and efficiency-corrected, was modeled using
Fig. 3. – Fit to the background-subtracted and efficiency-corrected χc1π mass distributions. See text for more details.
the Kπ mass distribution and the corresponding normalized Kπ Legendre polynomial moments. Figure 3 shows the results of the fits to the χc1π+ mass spectrum. The fit shown in fig. 3(a) includes both Z1(4050)+ and Z2(4250)+ resonances and the fit
shown in fig. 3(b) includes a single broad Z(4150)+ resonance. Figures 3(c,d) show
the χc1π mass spectrum fitted in the Dalitz plot region 1.0≤ m2(Kπ) < 1.75 GeV2/c4 in order to make a direct comparison to the Belle results [6] (this region is labeled as “window” in table II). The results of the fits are summarized in table II and in every case the yield significance does not exceed 2σ. The ULs on the 90% CL on the branching fractions are: B( ¯B0→ Z1(4050)+K−)× B(Z1(4050)+→ χc1π+) < 1.8× 10−5;B( ¯B0→
Z2(4250)+K−)× B(Z2(4250)+→ χc1π+) < 4.0× 10−5 andB( ¯B0→ Z+K−)× B(Z+→
χc1π+) < 4.7× 10−5.
TableII. – Results of the χc1π fits. Nσ and Fraction give, for each fit, the significance and the
fractional contribution of the Z resonances.
Data Resonances Nσ Fraction (%) χ2/NDF
a) Total Z1(4050)+ 1.1 1.6± 1.4 57/57 Z2(4250)+ 2.0 4.8± 2.4 b) Total Z(4150)+ 1.1 4.0± 3.8 61/58 a) Window Z1(4050)+ 1.2 3.5± 3.0 53/46 Z2(4250)+ 1.3 6.7± 5.1 b) Window Z(4150)+ 1.7 1.37± 8.0 53/47
invariant-mass distribution from the Y(4260) signal region.
5. – Study of the J/ψπ+π− system via initial state radiation (ISR)
The Y(4260) charmonium-like resonance was discovered by BaBar [8] in ISR pro-duction of J/ψπ+π−. A subsequent Belle analysis [9] of the same final state suggested
also the existence of an additional resonance around 4.1 GeV/c2 that they dubbed the
Y(4008).
The BaBar analysis of the J/ψπ+π− system produced in ISR has been repeated
using a data sample of 454 fb−1 [10].
The J/ψπ+π− mass region below ∼ 4 GeV/c2 is investigated for the first time. In that region an excess of events has been observed and the conclusion, after a detailed study of the ψ(2S) lineshape (to estimate the ψ(2S) tail contribution to that region), is that it is not possible to discount the possibility of a contribution from a J/ψπ+π−
continuum cross section in this region. From this study we obtain the cross section value 14.05± 0.26 (stat) pb for radiative return to the ψ(2S) and a measurement of the width Γ(ψ(2S) → e+e−) = 2.31± 0.05(stat) keV. Figure 4(a) shows the fit to
the J/ψπ+π− distribution. A clear signal of the Y(4260) is observed for which the values obtained are m[Y (4260)] = 4244± 5 ± 4 MeV/c2, Γ[Y (4260)] = 114+16
−15± 7 MeV and Γee× B(J/ψπ+π−) = 9.2± 0.8(stat) ± 0.7(syst) eV. No evidence for the state at ∼ 4 GeV/c2 reported by Belle [9] was seen. A study of the π+π− system from the
Y (4260) decay to J/ψπ+π−has been performed. The dipion system is in a predominantly
S-wave state. The mass distribution exhibits an f0(980) signal, for which a simple model
indicates a branching ratio with respect to J/ψπ+π− of 0.17± 0.13(stat). The fit to the
dipion invariant-mass distribution is shown in fig. 4(b). REFERENCES
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